A typical antenna includes at least one conductive antenna arm connected through a transmission line to a receiver, transmitter or transceiver. To transmit a radio frequency (RF) signal, a transmitter (or the transmitter portion of a transceiver) applies an oscillating RF current to the antenna arm, and the antenna arm radiates the energy from the oscillating current onto the “air interface” as electromagnetic waves. To receive a signal, the antenna arm converts electromagnetic waves that impinge upon the antenna arm from the air interface into voltages, which are provided to a receiver (or the receiver portion of a transceiver).
Half-wave dipole antennas and quarter-wave vertical antennas are among the most commonly implemented types of antennas, and they may be designed to operate within a desired bandwidth with a specific center frequency. Often, influences external to the antenna may cause the operating bandwidth of the antenna to shift. For example, when the antenna is incorporated into a system, the proximity of other system components to the antenna may affect the center frequency of the operating band. When those influences are predictable, they may be accounted for in the antenna design. However, when those influences are not predictable, they may cause the center frequency of the operating band to shift in an undesirable manner when the antenna is incorporated into a system.